Probably one of the first places that would be actively involved if another 9/11 type of situation came up would be the North American Aerospace Defense Command (NORAD) at Peterson Air Force Base in Colorado Springs, Colo. [Editor's note:The original version of this story listed the wrong location for Peterson Air Force Base.]

"NORAD remains responsible for defending the US and Canada from air attacks on our nations,” says spokesman John Cornelio, adding that NORAD can “unilaterally” make a decision based on its information and launch fighter jets to go after a potential threat.

The Federal Aviation Administration now shares its radar with NORAD, and both groups share representatives who serve as liaisons, says FAA Laura Brown. “All of the agencies involved developed a better relationship since 9/11,” she adds.

In an emergency, NORAD can scramble military jets armed with air-to-air missiles. In the case of the NYPD, it’s not clear what weapons it has at its disposal.

On Monday, in response to questions from reporters, Mayor Michael Bloomberg said, “The NYPD has lots of capabilities that you don’t know about and you won’t know about.”

Some news reports suggested the police might be able to mount a .50 caliber gun on a helicopter to shoot at a plane. Whether that would work is not clear.

But should the New York police be the ones making a decision to bring down an airliner, even if they are capable of it?

Terrorism experts have their doubts.

“There is no police department that should be making such decisions,” says Maki Haberseld, an expert on terrorism at John Jay College in New York. “The decision should be in the hands of the president of the country.”

She reasons that the decision to shoot down an airplane should be made by an elected official. “Police chiefs are appointed by mayors, they have not necessarily gone through some kind of election. It’s truly a scary perspective that someone like a police chief would be given that much discretion.”

Former FBI agent Mr. Defenbaugh also casts doubt on Kelly’s assertions. “I don’t know who is giving him the authority to knock down airplanes,” he says. “Not even in New York City.”